Many of the one-dimensional gratings used in optics have a more or less constant pitch across the two-dimensional surface area of the grating. This is true, for example, for the diffraction gratings used for in spectrometers and monochromators and for the gratings used in most distributed feedback (DFB) and distributed Bragg reflector (DBR) laser structures. For the spectroscopic application, the angular dependence of the scattering from the grating is described by the familiar grating equation: sin θout=sin θin+iλ/d where λ is the optical wavelength, d is the (nominally constant) period of the grating teeth, i (=0, ±1, ±2, . . . ) is an integer and θn (θout) is the incident (scattered) angle measured from the normal to the grating. Here it is assumed that the angles of incidence and scattering are in a plane perpendicular to the grating lines. For the laser application, the Bragg condition, n1d1+n2d2=λ/2, where n1 (n2) is the modal refractive index of the laser structure under medium 1(2) and d1(d2) is the thickness of medium 1(2), determines the lasing wavelength.
For the case of DFB and DBR lasers, the grating period is usually nominally constant along the lasing direction (across the entire laser cavity for DFB, just at the ends for DBR). There have been reports of longitudinal chirps to improve the resistance of the laser to hole-burning stabilities at high operating powers [P. Zhou and G. S. Lee, “Phaseshifted distributed feedback laser with linearly chirped grating for narrow linewidth and highpower operation,” Appl. Phys. Lett. 58, 331-333 (1991)], which is herein incorporated by reference.
A large tunable range for a chirped grating on a large-area, optically-pumped semiconductor laser structure has been recently demonstrated where the pump stripe was shifted relative to the chirped grating to provide the tuning mechanism. This was primarily a transverse chirp (e.g. the grating lines are splayed so that the spacing between adjacent grating lines changes as the grating is sampled along the lines. For longitudinal chirp, the spacing varies across a single line. The distinction between longitudinal and transverse chirp is illustrated schematically as prior art in FIG. 1.
FIG. 2 shows a prior art grating fabrication method of U.S. Pat. No. 7,656,912, which is commonly owned by the present assignee and that has an inevitable mixture of both transverse chirp (desirable for tuning) and longitudinal chirp (undesirable if the chirp is large enough so that the entire length of the pumped area cannot contribute to the lasing). In the optical scheme of FIG. 2, once the positions of the lens and the sample and the lens focal length are determined, both the longitudinal and the transverse chirps are fixed by the optical configuration. In FIG. 2, a coherent radiation source, for example a laser operable to produce radiation at 355 nm, directs radiation to a simple lens system 310, 320, and 330 that is used to control the wavefronts, e.g., to form a large area plane wave and to split that single large area plane wave into two symmetric smaller-area spherical waves. These waves are then incident on a photoresist coated sample wherein the interference between the two spherical waves is recorded in the photoresist by interferometric lithography. The interference between the two coherent spherical waves can produce a periodic spatially-varying intensity pattern. Such pattern can be transferred into, e.g., a top clad layer of a quantum well laser structure and thus form the plurality of grating lines in the semiconductor by an etching step. It is noted that the interference between two spherical waves produces both a transverse chirp (e.g. the grating lines are splayed relative to a single frequency grating) and a longitudinal chirp (the spacing between grating lines varies in a direction nominally perpendicular to the grating lines).
Therefore, a purpose of the current invention is to provide a method and apparatus for fabrication of a chirped grating wherein the longitudinal and transverse chirps can be independently controlled, at least over some range. In particular the goal is to reduce the longitudinal chirp for a fixed transverse chirp.